1. Field of the Invention
The present invention relates to cavity-backed antennas and to close-packed arrays of such antennas. The invention relates particularly to cavity-backed spiral antennas and especially to close-packed divergent arrays of such antennas when mounted near the forward tip of a pointed radome and incorporated in an amplitude-comparison monopulse radar system.
2. Description of Related Art
Cavity-backed spiral antennas operating over large radio frequency bandwidths are currently available with cylindrical cavities which are filled with radar absorbent material (RAM) and terminated by a balun box, and are used in monopulse radar systems. In an amplitude comparison configuration, in which the antenna axes diverge from the boresight, the diameter of the array is defined by the lowest frequency to be detected since this frequency determines the maximum spiral diameter required, by the size of the cavity, which must be sufficient to provide absorption of substantially all of the reverse-radiated emission from the spiral, and by the size of the balun box. For reasons which are explained below, it is desirable to minimize this diameter so that the array can be mounted as close as possible to the forward tip of a pointed radome, at the nose of a missile for example. However for a given bandwidth the diameter of the array is largely determined by the size, i.e. the depth, of each cavity. There is little scope for reducing the cavity depth because of the requirement to absorb the reverse-radiated emission from the spiral antenna (which would otherwise interfere with the forward beam).
Thus it has not been possible, hitherto, to mount arrays of cavity-backed antennas close to the forward tip of a streamlined radome housing, and consequently a serious problem arises. Since the radiating faces of the cavity-backed antenna face the inner surface of the surrounding radome and are typically separated from this surface by only a few millimeters, the respective divergent axes of the antennas are necessarily substantially normal to the radome surface. Consequently the antenna axes diverge from the boresight by an angle of typically 70.degree., so that the forward view performance of the array is poor because target return signals from the boresight direction are badly distorted by virtue of their large angle of incidence at the antennas. It is not practicable to reduce the divergence of the antenna axes by making the radome nose blunter, because the aerodynamic performance of the radome is then reduced and results in significant extra drag.